Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for balancing a load experienced by individual vibratory source elements that form a vibrator source array and/or for shaping a beam emitted by the source array.
Discussion of the Background
Reflection seismology is a method of geophysical exploration to determine the properties of a portion of a subsurface layer in the earth, information that is especially helpful in the oil and gas industry. Marine reflection seismology is based on the use of a controlled source that sends energy waves into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to estimate the depth and/or composition of the features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
For marine applications, sources are mainly impulsive (e.g., compressed air is suddenly allowed to expand). One of the most used sources is airguns which produce a high amount of acoustic energy over a short time. Such a source is towed by a vessel either at the water surface or at a certain depth. The acoustic waves from the airgun propagate in all directions. The emitted acoustic waves' typical frequency range is between 6 and 300 Hz. However, the frequency content of the impulsive sources is not fully controllable, and different sources are selected depending on the needs of a particular survey. In addition, the use of impulsive sources can pose certain safety and environmental concerns.
Thus, another class of sources that may be used are vibratory sources. Vibratory sources, including hydraulically-powered, electrically-powered or pneumatically-powered sources and those employing piezoelectric or magnetostrictive material, have been used in marine operations. However, there is no large-scale use of such sources because they have limited power and are unreliable due to the number of moving parts required to generate seismic waves. A positive aspect of vibratory sources is that they can generate signals that include various frequency bands, commonly referred to as “frequency sweeps.” In other words, the frequency band of such sources may be better controlled, as compared to impulsive sources.
One example of such a vibratory source is described in U.S. patent application Ser. No. 13/415,216 (herein the '216 application), filed on Mar. 8, 2012, and entitled “Source for Marine Seismic Acquisition and Method,” assigned to the same assignee as the present application, the entire content of which is incorporated herein by reference.
However, when combining plural vibratory source elements to form a vibratory source array, in order to generate more energy, the load on each vibratory source element increases. It is known that the dominant load a vibratory source element (sometimes called an acoustic projector) sees is the radiation impedance. At low frequencies, this is dominated by what looks like a mass loading. When more than one vibratory source element is used (i.e., a vibratory source array), then an additional load called mutual impedance is created by the effect of other vibratory source elements in the array. Also, ghost reflections from the air/water boundary can come into play.
The loading effect of mutual impedance is of greatest concern for the case when the overall dimension of the source array is small (typically, less than one wavelength in size). For arrays of large dimension (useful for beam-forming, i.e., to direct acoustic energy in a preferred direction), the mutual impedance loading is of less concern. With large arrays the source elements tend to be spread farther apart, so the mutual impedance loading tends to be less of an issue.
Thus, it is desirable to judiciously locate the array elements so the total load impedance each source element sees can be better balanced to maximize total output. Accordingly, it would be desirable to provide systems and methods that minimize load impedance for each vibratory source element and maximize energy output to overcome the afore-described problems and drawbacks.